Project Summary One of the most common medical conditions in any aging society is presbycusis, or age-related hearing loss. Approximately one third of American adults suffer from this condition typically starting at middle age, and about half of adults over 70 years have a substantial hearing impairment. One of the mechanisms of presbycusis happens in the central nervous system and is termed central hearing loss. Older adults with central hearing loss may have normal or near normal audiograms yet have problems carrying on a conversation in complex environments, i.e. situations where multiple sound sources are active at the same time, such as a busy restaurant, a public place, or any situation where background noises are active. The main reason for this difficulty is that affected individuals have trouble perceptually isolating sound sources of interest (e.g., the voice of the speaker they want to listen to) effectively from other sources, presumably because the neural mechanisms that perform this computation are less effective. We propose a combined human subjects and animal model study to tackle this issue. Specifically, we propose to study the contribution of age-related changes in the sound localization pathway, located in the auditory brain stem, to this phenomenon. The overarching hypothesis of this proposal is that precise timing of neural activity within the sound localization pathway declines with age, leading to less precise binaural hearing and sound localization abilities. We hypothesize that these changes contribute to decreased behavioral performance in complex acoustic environments in both humans and animal models. For the human component, we will test the sound localization and hearing in noise abilities of ?young? and ?old? listeners, and relate decreased behavioral performance in older listeners to decreased temporal precision of neural activity in the sound localization pathway (aim 1). We will show that aged Mongolian gerbils recapitulate these age-related changes, making them good models to study this human condition (aim 2). We will then explore cellular mechanisms behind the decreased temporal precision of neural activity (aim 3). The expected results from this study will help determine the role of the sound localization pathway in this common medical condition, will help diagnose central hearing loss more objectively, will directly suggest mechanisms for the development of future medical interventions, and will help resolve a key challenge to healthy aging.